Things You'll Need
Right angle grinder with wire wheel
Large propane or acetylene torch
Large ball peen hammer and anvil or strike plate
Mower blades become bent during heavy use, whether it is from hitting one rock too many or constantly being scraped over uneven ground. Replacement blades are expensive, so it is no wonder many people try to bend them in a vise or hammer them straight. This is frustrating, since most mower blades are tempered spring steel. The answer lies in the blacksmith's shop: heat it and beat it.
Remove your lawn mower blade. Use a putty knife to scrape any grass or vines off the blade.
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Clean your mower blade with a wire wheel on a right-angle grinder. Scuff the bent section with a 24-grit wheel until you can see shiny metal.
Hold the far end of the blade in vise grips. Heat the bent area to royal blue with a torch.
Use your choice of hammer and an anvil or strike plate to straighten the blade while it is still royal blue, or put the blade in a bench vise and bend it. The steel is in a relaxed condition at that color and will straighten fairly easily.
Scuff the entire blade with a 24-grit wheel on your right angle grinder. Reheat the entire blade to royal blue, and allow it to air cool slowly. This is called stress relieving. According to Gypsy Wilburn, "It relaxes the metal fatigue caused by the initial bend as well as the fatigue caused by reforging the blade."
Sharpen the blade on each side so that it is balanced. Reinstall the blade.
According to Grounds Maintenance.com, when you resharpen the blade, the "... primary goal is to consistently maintain the correct angle on the blade. Around 40 degrees is typical, but this can vary, so check with the blade manufacturer to obtain the exact figure."